On the Need for Additional Evidence

There is an interesting new report from the University of Waterloo that claims Chlorofluorocarbons (CFCs) are the chief cause of global warming since the 1970s. It was published in the International Journal of Modern Physics B  last week. As phys.org explains,

CFCs are already known to deplete ozone, but in-depth statistical analysis now shows that CFCs are also the key driver in global climate change, rather than carbon dioxide (CO2) emissions.

"Conventional thinking says that the emission of human-made non-CFC gases such as carbon dioxide has mainly contributed to global warming. But we have observed data going back to the Industrial Revolution that convincingly shows that conventional understanding is wrong," said Qing-Bin Lu, a professor of physics and astronomy, biology and chemistry in Waterloo's Faculty of Science. "In fact, the data shows that CFCs conspiring with cosmic rays caused both the polar ozone hole and global warming."

"Most conventional theories expect that global temperatures will continue to increase as CO2 levels continue to rise, as they have done since 1850. What's striking is that since 2002, global temperatures have actually declined – matching a decline in CFCs in the atmosphere," Professor Lu said. "My calculations of CFC greenhouse effect show that there was global warming by about 0.6 °C from 1950 to 2002, but the earth has actually cooled since 2002. The cooling trend is set to continue for the next 50-70 years as the amount of CFCs in the atmosphere continues to decline."

I might not be the most qualified person to criticize Dr. Lu's work, but I do think we should not jump to conclusions right away. The carbon/ temperature record goes back a long time, tens of thousands of years if we are referring to ice cores.With that much data, it's fair to say that the very strong correlation between carbon and temperature can be assumed to continue to the future. But even with that much data, establishing a causal basis can be very difficult. Since we can't run control experiments, we'll almost always have enough uncertainty in the data to keep climate denier websites busy for decades.

The black line represents a three-year moving average. Data from the NOAA, which is available here.

Considering this challenge, I find it very presumptuous to go that far against consensus based on only 10 years' worth of deviation from the original trend. The argument for CFCs is based entirely on the fact that temperatures have not risen as quickly as CO2 levels in the last decade, even though it remains much hotter than any period before it. Considering measurement challenges and the huge number of almost random processes that influence weather (just ask Lorenz about that), it seems presumptuous, to say the least, to claim that CFCs are the only compounds that matter. There's just too much that we don't understand to make that claim with that little of evidence.

To be fair, I'm not claiming that CFCs don't matter in this debate. I only want to recommend caution about extreme claims. A much older report from the US Environmental Protection Agency did say CFCs were responsible for about 15 percent of global warming. They could also play a strong role in the less-than-expected recent rise in temperatures. So could volcanoes, so could a lot of other things.

So what's really going here? Academics have strong incentives to make almost absurd claims, since it helps them get published, and it generates references. While this can be annoying on a case by case basis, the overall scientific discourse benefits from the trolls as it forces people to expand their cases and move the debate forward.  It will take a lot to overturn consensus at this point, since about 97 percent of scientists in a recent survey agree on the mechanism and the effect. As readers of scientific literature, wish should keep this in mind. Prior beliefs should be adjusted only when there is substantial evidence to change course. So far with CFCs and global warming, we're not there yet.